[SDAG] Use shifts if ISD::MUL is illegal when lowering ISD::CTPOP

We can avoid libcalls.

Fixes #86205

Author: wangpc-pp

llvm/lib/CodeGen/SelectionDAG/TargetLowering.cpp

@@ -8709,11 +8709,21 @@ SDValue TargetLowering::expandCTPOP(SDNode *Node, SelectionDAG &DAG) const {
                      DAG.getConstant(0xFF, dl, VT));
   }
 
-  // v = (v * 0x01010101...) >> (Len - 8)
-  SDValue Mask01 =
-      DAG.getConstant(APInt::getSplat(Len, APInt(8, 0x01)), dl, VT);
-  return DAG.getNode(ISD::SRL, dl, VT,
-                     DAG.getNode(ISD::MUL, dl, VT, Op, Mask01),
+  SDValue V;
+  if (isOperationLegalOrCustomOrPromote(ISD::MUL, VT)) {
+    // v = (v * 0x01010101...) >> (Len - 8)
+    SDValue Mask01 =
+        DAG.getConstant(APInt::getSplat(Len, APInt(8, 0x01)), dl, VT);
+    V = DAG.getNode(ISD::MUL, dl, VT, Op, Mask01);
+  } else {
+    V = Op;
+    SDValue ShiftC = DAG.getConstant(8, dl, VT);
+    for (unsigned I = 8; I < Len; I += 8) {
+      V = DAG.getNode(ISD::ADD, dl, VT, Op,
+                      DAG.getNode(ISD::SHL, dl, VT, V, ShiftC));
+    }
+  }
+  return DAG.getNode(ISD::SRL, dl, VT, V,
                      DAG.getConstant(Len - 8, dl, ShVT));
 }
 

llvm/test/CodeGen/LoongArch/ctlz-cttz-ctpop.ll

@@ -285,9 +285,12 @@ define i32 @test_ctpop_i32(i32 %a) nounwind {
 ; LA64-NEXT:    lu12i.w $a1, 61680
 ; LA64-NEXT:    ori $a1, $a1, 3855
 ; LA64-NEXT:    and $a0, $a0, $a1
-; LA64-NEXT:    lu12i.w $a1, 4112
-; LA64-NEXT:    ori $a1, $a1, 257
-; LA64-NEXT:    mul.d $a0, $a0, $a1
+; LA64-NEXT:    slli.d $a1, $a0, 8
+; LA64-NEXT:    add.d $a1, $a0, $a1
+; LA64-NEXT:    slli.d $a1, $a1, 8
+; LA64-NEXT:    add.d $a1, $a0, $a1
+; LA64-NEXT:    slli.d $a1, $a1, 8
+; LA64-NEXT:    add.d $a0, $a0, $a1
 ; LA64-NEXT:    bstrpick.d $a0, $a0, 31, 24
 ; LA64-NEXT:    ret
   %1 = call i32 @llvm.ctpop.i32(i32 %a)